Richard L. Hyson
Program in Neuroscience
Florida State University
Neurons in the avian nucleus laminaris (NL) are the first to receive binaural information and are presumed to play a role in encoding interaural time differences (ITD), an important cue for sound localization. Models of neural networks which code ITDs commonly postulate a circuit consisting of different "delay lines" projecting onto an array of "coincidence detectors." Electrophysiological recordings from a brain slice preparation have shown that both of these features are present in the chicken brain stem auditory system. First, extracellular recordings from the chick NL have shown that the arrival of information from the contralateral cochlear nucleus, n. magnocellularis (NM) is systematically delayed across the medial-to-lateral extent of NL. Second, both extracellular and intracellular recordings from NL neurons have shown that the likelihood of generating an action potential is dependent on the timing of inputs arriving from NM on the two sides of the brain.
Although modeling of ITD coding in this circuit has been based primarily on how NL neurons detect coincidence of their two excitatory inputs, it is important to note that these neurons are also contacted by terminals containing the inhibitory neurotransmitter, GABA. Unexpectedly, however, GABA can both increase and decrease the excitability of NL neurons. Using the brain slice preparation, both the ipsilateral and contralateral excitatory inputs to NL were electrically activated and the delay between trains of bilateral stimuli (simulated-interaural time difference [s- ITD]) was varied. The resulting S-ITD response functions were recorded in the presence of various doses of GABA. GABA had different effects on the s-ITD functions depending on the drug concentration. A low GABA dose enhanced excitability at favorable s-ITD, but not at unfavorable s-ITDs. In contrast, higher GABA concentrations diminished excitability. Moderate GABA concentrations had no consistent effect. A similar bi-directional effect of GABA was observed in NM. These results suggest that the GABAergic input to the brain stem auditory system will either increase or decrease the excitability of this circuit depending on the degree to which this GABAergic input is activated. It is possible that GABA might dynamically adjust the excitability of these neurons, allowing for optimal coding of ITDs across stimulus intensities. (Supported by NIDCD grant DC00858).
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